1. Field of the invention
The present invention relates to a glazing unit comprising a composite glazing panel having a pair of vitreous sheets adhered to an intervening layer of polymeric material sandwiched therebetween.
A glazing panel which consists of or includes a pair of vitreous sheets adherent to an intervening layer of polymeric material, is referred to herein as a composite panel.
References to acoustic properties of composite panels throughout this specification are references to such properties measured in accordance with the German Federal Republic Standard VDI 2.719.
2. Description of the Related Art
It is known from British patent specification GB 2227207-A (Glaverbel) to provide a composite glazing panel which consists of or includes a pair of vitreous sheets adherent to an intervening layer of polymeric material, wherein the sheets of said pair are separated by a said intervening layer of polymeric material which layer has viscoelastic properties such that the critical frequency of coincidence (or the lower or lowest critical frequency of coincidence if there is more than one) of the panel ["the coincidence frequency (.phi..sub.p) of the panel"] is greater than the critical frequency of coincidence of a notional monolithic vitreous sheet ["the coincidence frequency (.phi..sub.m) of the equivalent monolith"] which monolithic sheet is of the same shape and area as the panel and has a mass equal to the total mass of vitreous material in the panel.
Such a composite panel as is known from British patent specification GB 2227207-A (Glaverbel) referred to above, has, by virtue of the nature of its interlayer, improved acoustic insulating properties as compared with an otherwise similar composite panel which does not have such an interlayer. A composite panel may be installed in a building, or in a vehicle. One use for which composite panels are especially suitable is as glazings for railway carriages, particularly those of underground railway systems or of high speed transit systems where for one reason or another, ambient noise levels may be rather high.
The present invention has especial reference to the use of such composite panels in double-glazed units, the composite panel having acoustic attenuation properties. Hollow glazing units can have very good acoustic insulation characteristics, and they also afford good thermal insulation.
The commercial demand is more and more to install the panels in glazing bays by adhesively securing the panels to the support structure instead of mounting the panels in a chassis having a U-section profile, in which the edges of the panel are retained. This recent technique of panel installation is often referred to as "exterior panel adhesive fixing" or (somewhat improperly) as "structural glazing". This technique allows one to form facades which appear to be totally glazed. It also allows one to install the panels in a flush manner with the body-work of a vehicle for example. This technique allows one to obtain advantageous aesthetic effects.
There are two general methods possible for the manufacture of such units. Firstly, the composite panel may be assembled together with a frame in the factory and then the assembled unit is secured in position at the building site or vehicle construction works, depending on the intended use of the unit. Alternatively, the unit is assembled directly at the building site or vehicle construction works, where the unit is fixed to a frame or to the supporting structure rather than being retained in a U-section chassis.
At an elevated temperature such as about 50.degree. C., for example in sunshine, the intervening polymeric material of the composite panel has a tendency to flow, since its modulus of elasticity generally varies with temperature. The external sheet of vitreous material, which is held to the rest of the structure by the intervening polymeric material therefore tends to become loose and may even fall out of the structure.